Introduction to Fungi, Third Edition

242 ASCOMYCOTA (ASCOMYCETES)
Fig 8.13 Xylaria longipes. Fine structure of the ascus apex
(after Beckett & Crawford,1973). (a) L.S. undischarged ascus
showing the apical ring. (b) L.S. discharged ascus showing the
eversion of the apical ring.
(Gr. para- ¼ near, beside, parallel; physis ¼
growth), or pseudoparaphyses. The general
term for such sterile inter-ascal tissue is the
hamathecium (Gr. hama ¼ all together, at the
same time) (Eriksson, 1981). Paraphyses are filaments
which are attached to the ascocarp near
the bases of the asci and are free at their upper
ends as in Pyronema (Figs. 14.2a,c) and Ascobolus
(Fig. 14.6). Pseudoparaphyses are hyphae which
usually arise above the level of the asci and grow
downwards between them. They may become
attached at their lower ends as in Pleospora
(Fig. 17.9). Because the paraphyses and pseudoparaphyses
pack tightly around the asci, the
latter cannot expand laterally but are forced to
elongate. A hamathecium is lacking in certain
groups of ascomycetes, e.g. the Eurotiales
and Clavicipitales, and also in Mycosphaerella
(Fig. 17.19). The sum of all contents of the
ascoma (i.e. the hamathecium plus asci) but
excluding the ascoma wall is called the centrum.
8.6.5 The mechanism of
ascospore discharge
Explosive release of ascospores follows increased
turgor pressure, caused by water uptake by the
ascus. In the young ascus, after the spores have
been cut out, the epiplasm remains lining the
ascus wall, and this surrounds a large central
vacuole containing ascus sap, within which
the ascospores are suspended. The epiplasm is
rich in the polysaccharide glycogen which can
be visualized cytochemically by its reddishbrown
staining with the I 2 /KI stain. As the ascus
matures, the red stain diminishes in intensity
due to the conversion of the polysaccharide to
osmolytes of lower molecular weight. This brings
about an increased osmotic concentration of
the ascus sap, followed by increased water
uptake. The resulting increase in turgor pressure
causes the ascus to stretch and, eventually,
to burst open, squirting out the ascospores.
The osmotic pressure of the sap in mature asci
extending from apothecia of Ascobolus immersus
has been determined to be up to 3 bar (0.3 MPa),
with glycerol being the main organic osmolyte
(Fischer et al., 2004). In Gibberella zeae, the turgor
pressure required for ascus discharge (1.54 MPa)
seems to be caused mainly by a K þ and Cl influx
across the plasma membrane, with the most
abundant organic osmolyte (mannitol) making
only a small contribution (Trail et al., 2005).
Higher turgor pressures have been recorded
when asci are mounted in water (Ingold, 1939,
1966).
In cup fungi (discomycetes), as the asci
mature they elongate and project above the